The long-range goal of the principal investigator is to examine the mechanisms that regulate the autonomic nervous system. The specific goals of this program are to better understand the mechanisms regulating interstitial NE concentrations ((NE)i during exercise in humans. NE release evokes smooth muscle contractions and vasoconstriction. Our first aim in this proposal is to examine in human subjects the effects of changes in the interstitial concentration of potassium, nitric oxide and ATP on (NE)i. We hypothesize that potassium, NO and ATP will raise (NE)i. We will then explore the mechanisms by which these substances mediate their postulated effects. Our second aim is to examine and compare (NE)i in contracting and resting muscle during one-legged exercise. We hypothesize the (NE)i will be greater in contracting than in non-contracting muscle. We will then attempt to locally and selectively modify the interstitial milieu in order to better understand the determinants of (NE)i during exercise. Preliminary data from our laboratory suggests that the (NE)i in skeletal muscle is approximately 5.5 times the plasma concentration and is likely to be far more reflective of events within the neurovascular junction than plasma NE determinations or measurements of NE spillover (determined with radiotracer methods). A key feature of this proposal is that we will use microdialysis techniques in human subjects to determine (NE)i. To the best of our knowledge, experiments such as these have never been performed in human subjects. The work will be performed in a NIH funded General Clinical Research Center (GCRC) housed in the College of Medicine at the Hershey Medical Center, Hershey, PA.